Replace django.core.urlresolves with django.urls
(In Django 2.0) The django.core.urlresolvers module is removed
in favor of its new location, django.urls.
It was deprecated in Django 1.10:
https://docs.djangoproject.com/en/2.0/releases/1.10/#id3
Note that this commit does not add py35dj20 like other horizon plugins
because there is no actual python unit tests in freezer-web-ui.
Older Django unit tests were dropped from tox.ini
as horizon dropped Django <=1.10 support in Rocky.
Change-Id: I38f3f05655324f2a5f38c6e8c6ab53ebaa42d4c8